The so-called “lost-foam” casting process is a well-known technique for producing metal castings wherein a fugitive, pyrolizable, polymeric, foam pattern (including casting, gating, runners and sprue) is covered with a thin (i.e. 0.25-0.5 mm), gas-permeable, refractory (e.g. mica, silica, alumina, alumina-silicate, etc.) coating/skin, and embedded in compacted, unbonded sand to form a pattern-filled, mold cavity within the sand. Molten metal (hereafter “melt”) is then introduced into the pattern-filled mold cavity to melt, pyrolyze, and displace the pattern with melt. Gaseous and liquid decomposition/pyrolysis products escape through the gas-permeable, refractory skin into the interstices between the unbonded sand particles. The casting rate (i.e. the rate at which the metal enters the mold cavity) is limited by the rate the advancing melt front can displace the pattern from the cavity, which, in turn, is affected by the thickness and permeability of the refractory skin/coating. Typical fugitive polymeric foam patterns comprise expanded polystyrene foam (EPS) for aluminum castings, and copolymers of polymethylmethacrylate (PMMA) and EPS for iron and steel castings.
The polymeric foam pattern is made by injecting pre-expanded polymer beads into a pattern mold to impart the desired shape to the pattern. For example, raw expandable polystyrene (EPS) beads (ca. 0.2 to 0.5 mm in diameter), containing a blowing/expanding agent (e.g. n-pentane), are: (1) first, pre-expanded at a temperature above the softening temperature of polystyrene and the boiling point of the blowing agent; and (2) then, molded into the desired configuration in a steam-heated pattern mold which further expands the beads to fill the pattern mold. Complex patterns and pattern assemblies are made by molding several individual mold segments, and then gluing them together to form the finished pattern/assembly.
The melt may be either gravity-cast (i.e. poured from an overhead ladle or furnace), or countergravity-cast (i.e. forced upwardly by vacuum or low pressure into the mold cavity from an underlying vessel, e.g. a furnace). In gravity-cast lost-foam processes, the metallostatic head of the melt in the sprue and pouring basin is the driving force for filling the mold cavity with melt. Gravity-cast, lost-foam processes are known that: (1) top-fill the mold cavity by pouring the melt into a basin overlying the pattern so that the melt flows downwardly into the mold cavity through a gating system (i.e. one or more gates) located above the pattern; (2) bottom-fill the mold cavity by pouring the melt into a vertical sprue that lies adjacent the pattern and extends from above the mold cavity to the bottom of the mold cavity for filling the mold cavity from beneath through a gating system located beneath the pattern so that the melt flows vertically upwardly into the mold; and (3) side-fill the mold cavity by pouring the melt into a pattern that forms a vertical sprue that lies adjacent the mold cavity, and communicates with the mold cavity via a plurality of vertically aligned runners and gates that horizontally fill the mold cavity from the side thereof. The vertical sprue may be flanked by two or more mold cavities for making multiple castings with a single pour.
Faster casting rates result in less heat loss from the melt. Less heat loss during pouring keeps the melt hotter, which, in turn, reduces the formation of “folds” (i.e. pyrolysis products trapped at the confluence of cold metal fronts), and “cold-shuts” (i.e. sites where metal that does not completely fill the pattern due to premature solidification) in the casting.
Casting rates have heretofore been increased by providing one or more melt flow-channels (i.e. foam-free shafts) that extend into the pattern, and through which the melt can rush into selected portions of the pattern. Such flow channels are often called “lighteners” (which term shall be used herein), and are typically formed in the pattern at the joints where individual pattern segments are glued together to form a complete pattern. Alternatively, for straight lighteners, the pattern may be molded around an insert (e.g. a rod) that is subsequently withdrawn from the pattern leaving the lightener. Lighteners can classified both as to their “type” (i.e. their configuration), and as to their “application” (i.e. their location in the pattern). For example, one “type”-classified lightener is a so-called “pencil”-lightener which is a long, slender, cylindrical or polygonal, foam-free shaft formed in the pattern. One “application”-classified lightener, for example, is a so-called “sprue”-lightener which is a lightener used in a sprue-forming portion of the foam pattern. See also “runner”-lighteners, for lighteners used in runner-forming portions of a pattern.
The side-fill lost foam process has heretofore been used commercially to manufacture cylinder heads for internal combustion engines. Patterns therefor have heretofore had a central, vertical, rectangular (ca. 4.3 cm.×4.1 cm.), sprue-forming portion (i.e. for forming a sprue in the compacted sand) flanked by a pair casting-forming mold cavities (i.e. for forming/shaping the heads), each coupled to the central sprue-forming portion by fourteen vertically arranged, runner-forming and gate-forming portions (i.e. for forming runners and gates respectively). Pencil-type lighteners extend the length of the sprue-forming portion (i.e. a sprue-lightener), and into the top four of the fourteen runner-forming portions (i.e. runner-lighteners). A pouring-basin-forming portion of the pattern forms a pouring basin that overlies the sprue-forming portion, and receives melt from a ladle, furnace etc. for gravity delivery to the sprue formed by the sprue-forming portion. A fugitive plug, made from the same foam as is used to make the pattern, is positioned between the pouring basin and the upper end of the sprue, and serves to delay outflow of melt from the basin into the sprue sufficient to provide enough residence time (i.e. 1-2 secs) for a prescribed amount of melt to accumulate in the pouring basin. The prescribed amount is sufficient to allow the melt therein to become quiescent, degas, and build-up a metallostatic head sufficient to cause the melt to gush forcefully out of the basin, and into the sprue, when the plug releases (i.e. evaporates), so as to quickly (ca. 1 sec) fill the sprue-lightener and force any air or pyrolysis gases that might otherwise be trapped in the sprue-lightener into the surrounding sand without creating any bubbles in the melt.